One of the best ways to plane a long and narrow piece (on a bench with a traditional tail vise) is to hold it in a drill press vise. I’ve glued pieces of 1/4” ply on the jaws of the drill press vice to keep it from marring the wood. This piece of wood is a little over seven feet long, and too long to hold with the regular bench dogs.
The bench needs to be dead flat so that the wood being planed isn’t distorted by something wrong with the bench. Using this method I can take a single shaving the full length with a smoothing plane.